I. Field of the Invention
This invention relates generally to material handling and inspection equipment and, more particularly, to a system for detecting an insufficient coating on coated metal sheet stock material.
II. Discussion of the Related Art
Sheet stock material is often laminated or coated with a layer of diverse composition to give it certain surface characteristics not found in the unprocessed material. Often, the stock material is inexpensive plastic or metal making up the bulk of a product and the coating is plastic for making the product safer or more appealing. Coating characteristics, such as electrical properties or chemical activity, or perhaps the look and feel of the coating, are often made to differ from those of the underlying material.
In the food container industry, and more specifically in the canned food industry, metal sheet stock is often used as the bulk material. The metal may be, for example, aluminum, steel or tin-plate converted into any size and shape container or lid. However, metal food containers must be coated to prevent contamination of the food inside the container and to prevent corrosion of the metal leading to failure of the container. The coating material is typically an enamel or lacquer paint. To provide an effective barrier, the enamel or lacquer coating must be continuous, i.e., must not contain voids or imperfections that permit the inner surface of the sheet stock to come into direct contact with food or the outer surface with the ambient environment. In addition, uncoated metal sheet stock is known to gall up the stamping dies used to form the containers. This increases the down time needed for cleaning dies and shortens die life significantly.
Detecting defects in the sheet material is preferably done before the bulk material is cut or formed into a specific product. For example, U.S. Pat. No. 3,188,478, issued to Binks, discloses an optical device for detecting pinholes in tin plate sheet material used to produce cans. Pinholes are typically very small and cannot be found by visual inspection. As the bulk sheet material passes through the detector, light from a source shining through a pinhole is used to actuate a photosensitive means which, in turn, causes a response in the form of a visible or audible signal. Of course, a device such as the one described by Binks cannot detect an insufficient coating on the surface of opaque sheet material.
In U.S. Pat. No. 3,759,095, Short, Jr. et al., describe a device for detecting surface flaws, such as cracks or splices, in a filmstrip. A mechanical feeler for sensing the surface of the film is attached to a piezoelectric crystal which moves in response to movement of the feeler. The piezoelectric crystal signals a circuit, which includes a capacitor for blocking gradually changing electric signals. Since only sudden changes in surface smoothness are detected, the device is not suited for detecting voids or insufficiently coated areas on a piece of sheet material. For example, the coating may be rough enough to give false positive signals or the transition from a coated portion to an uncoated portion may be smooth enough to produce no signal at all.
Devices and methods for measuring the thickness of a coating based on the electrical nature of the coating or based on the interaction of high energy radiation with the coating have also been described. U.S. Pat. No. 4,780,680, issued to Reuter et al., concerns an approach wherein the coating is charged by means of a corona and retained charge levels are measured by an electrostatic volt meter positioned downstream. These retained levels are then related to the thickness of the coating. U.S. Pat. No. 4,451,732 describes a very precise measuring system employing a radiation source and a Geiger-Muller Tube for detecting reflected radiation. Beta radiation is reflected from the coated sheet material and the intensity of the reflected radiation is measured to determine the thickness of the coating. However, radiating coated sheet material in the food container industry may not be acceptable and the precision available with this type of measurement is not needed.
Coating characteristics such as conductivity may also be measured directly and if the coating and the sheet material have different conductivities, a void or near void may be detected. U.S. Pat. No. 5,001,433, issued to Osaki, describes an apparatus and method for measuring the conductivity of material using a wave guide tube in which one end is connected to a transmitter for introducing a microwave and the other end open and directed toward the material. Either the reflected waves or the waves propagating through the material may be detected and measured to arrive at a determination of the conductivity.
Electrical conductivity is also measured by simply contacting the coated surface with multiple spaced contacts having different voltages and detecting and indicating shorts between contacts. It may also be configured to detect open circuit conditions, if desired. This type of device provides the necessary information at a cost which is typically below that of the radiation, corona charging or wave guide techniques.
One device employing this principle is an enamel void detector for can lines using coated sheet material produced by Opto-Mech, Inc. The Opto-Mech system includes a top and bottom roller each having 78 individual spaced detector rings. The rollers rotate as the sheet material passes between them and the rings float independent of adjacent rings to maintain contact with the sheet material. Every other ring is electrically connected together and also connected to a positive or a negative voltage such that adjacent rings have different potentials. The shorting of adjacent rings together through direct contact with metal sheet material, for example, indicates a void. Proximity switches sense the leading and trailing edges of the sheet material to prevent false triggering of the system due to uncoated edges. The electrical system uses brushes, such as carbon brushes, to provide contact between the rings and external electronics designed to compensate for changes in sensitivity; however, the brushes are prone to pick up dirt and dust from the sheet material and this may erode signals sent to the electronics.
The present invention solves problems associated with prior multiple spaced conductor type systems in a manner that increases reliability by enabling reduced sensitivity. The system enables conductivity probes to be wired directly to an electrical detection circuit and the detection circuit which may be relatively simple compared to those in other systems.
A principal object of the present invention is to provide a reliable system for detecting coating defects on a piece of coated sheet material of interest.
Another object of the invention is to provide a reliable system for detecting an insufficient coating on a piece of material of interest, based on detecting differences in conductivities between the material and the coating as detected by an electronic circuit.
Still another object of the invention is to provide an electrical system for detecting voids in a non-conductive coating on conductive sheet material that is relatively uncomplicated both mechanically and electrically.
A further object of the invention is to provide an improved system for detecting voids in a non-conductive coating on the surface of metal sheet material used in the fabrication of metal food containers.
A still further object of the invention is to provide an improved system for avoiding false signals due to uncoated edges of the material.
A yet still further object of the invention is to provide an improved system for detecting an insufficient coating on a sheet of material that also detects bent edges.
Other objects, features and advantages of the present invention will become apparent to those skilled in the art through familiarity with the specification, claims and drawings herein.